Sidewinder, like all other Shingu, was created four hundred years before the start of the series by the Emperor of that time in an attempt to create weapons that could surpass the Teigu.
It was kept hidden in the Empire's secret storage until It was given to Green by Gozuki.
Sidewinder is a Shingu that moves in accordance to its possessor's will. It's strong characteristics can make it difficult to maneuver.
A close-up of Sidewinder's handle
Sidewinder being used to attack multiple enemies
Sidewinder is a nickname given to a pit-viper species, the Crotalus cerastes and a venomous viper species, the Bitis peringueyi, both of said species live in deserts, with the former having an unusual movement patterns and the latter being an ambush hunter. This is interesting if you consider sidewinder's unusual and hard to predict movements.
↑ 1.01.1Akame ga Kill! Zero manga; Chapter 3, page 11
↑ 2.02.12.2Akame ga Kill! Zero manga; Chapter 3, page 20